Vacuum circuit breaker with arc rotation contact means



A. N. GREENWOOD Dec. 21, 1965 VACUUM CIRCUIT BREAKER WITH ARC ROTATION CONTACT MEANS Filed March 16, 1964 nnnm /8 /N VENT 0f?! ALLA/V GREENWOOD, 5y ALMA... 3 2mm ATTORNEY United States Patent Ofiice 3,225,167 Patented Dec. 21, 1965 3,225,167 VACUUM CIRCUIT BREAKER WITH ARC ROTATION CONTACT MEANS Allan N. Greenwood, Media, Pa., assignor to General Electric Company, a corporation of New York Filed Mar. 16, 1964, Ser. No. 351,937 8 Claims. (Cl. 200-144) This invention relates to electrode structure for an electric circuit interrupter and, more particularly, to electrode structure which is especially, though not exclusively, suited for use in circuit interrupters of the vacuum-type.

The usual vacuum type circuit interrupter comprises a pair of separable electrodes, or contacts, disposed within an evacated chamber. Circuit interruption is initiated by separating these electrodes to establish an arc. Assuming that the circuit is an alternating current circuit, the arc maintains itself until about the time a natural current Zero is reached, after which the arc is prevented from reigniting by the high dielectric strength of the vacuum.

It has been recognized heretofore that the interrupting capacity of such an interrupter can be materially increased by rotating the terminals of the arc at high speed along the surfaces of the electrodes. Such rotation tends to minimize the amount of metallic vapors generated from the electrodes by the arc and tends also to increase the degree of diffusion of the vapors that are generated. These factors enable the vacuum to recover its dielectric strength at an increased rate after a current zero and thus render the vacuum more capable of preventing reestablishment of the arc during this critical interval.

In types of interrupters other than those of the vacuum-type, high speed arc rotation also facilitates circuit interruption by cooling the arc column, decreasing electrode heating and vaporization, and promoting an increased rate of dielectric recovery.

A type of arc-rotating scheme that has been found satisfactory for vacuum interrupter applications is the type in which the electrodes of the interrupter contain slots for directing the current to follow certain paths that result in establishment of magnetic forces for are rotation. Most of these slotted-type electrodes are relatively complex and expensive to make because they contain either a large number of slots or slots of complex configuration.

An object of my invention is to provide a slotted type electrode that is capable of developing a high magnetic force for producing arc rotation and yet requires only a single slot that may be of a simple configuration.

Another object is to provide a simple slotted electrode for arc rotation that is able to withstand very high mechanical forces without being damaged.

Still another object is to provide a simple and rugged arc-rotating electrode that is free of regions where there is no arc-rotating magnetic force.

In carrying out my invention in one form, I provide an electric circuit interrupter that comprises a pair of electrodes between which an arc can be developed. One of the electrodes comprises a tubular member of relatively high conductivity metal. This tubular member has a pair of end surfaces extending transversely of the longitudinal axis of the tubular member and located in spacedapart relationship along the longitudinal axis. One of these end surfaces faces the other of the electrodes and constitutes an arc-running surface for arcs developed between the electrodes, while the other end surface constitutes a back surface. Current-directing means is provided for forcing the net current flowing through the tubular member to an arc terminal located at any point on the arc-running surface to follow a path through the tubular member that has a component extending circumferentially of the tubular member. This current-directing means comprises: (1) a slot in the tubular member extending between the two end surfaces and intersecting the respective end surfaces at points circumferentially spaced from each other and (2) means for forcing the major portion of the current flowing through said tubular member to an arc terminal at any point on said arc-running surface to enter said tubular member through an entry region located adjacent the point where the slot intersects the back surface and on the opposite side of the slot from the location of any point on the arc-running surface that is circumferentially aligned with the entry region. The electrode also includes means for supporting the tubular member comprising a plurality of elements of relatively low conductivity material circumferentially-spaced about the tubular member and joined to said tubular member at the back surface.

For a better understanding of my invention, reference may be had to the following description taken in conjuction with the accompaying drawings, wherein:

FIG. 1 is a cross-sectional view through a vacuumtype circuit interrupter embodying one form of my invention. The circuit interrupter comprises separable elec trodes shown in a closed or engaged position.

FIG. 2 is an enlarged perspective view of the electrode structure contained in the interrupter of FIG. 1. The electrodes are shown in a separated position, the spacing between the electrodes being exaggerated to facilitate a clear illustration of the electrode structure.

FIG. 3 is a sectional view through a portion of the electrode structure of FIG. 2.

FIG. 4 is a sectional view through another portion of the electrode structure of FIG. 2.

FIG. 5 illustrates a slightly modified electrode structure.

FIG. 6 illustrates another modification of the electrode structure.

Referring now to the interrupter of FIG. 1, there is shown a highly evacuated envelope 10 comprising a casing 11 of suitable insulating material and a pair of metallic end caps 12 and 13 closing off the ends of the casing. Suitable seals 14 are provided between the end caps and the casing to provide vacuum-tight joints at these points.

Located within the envelope 10 is a pair of separable electrodes, or contacts, 17 and 18 shown in their engaged or closed-circuit position. The upper electrode 17 is a stationary electrode suitably secured to a conductive rod 17a, which at its upper end is integrally united to the upper end cap 12. The lower electrode 18 is a movable electrode joined to a conductive operating rod 18a which is suitably mounted for vertical movement. The operating rod 18a projects through an opening in the lower end cap 13, and a flexible metallic bellows 20 provides a seal about the rod 18a to allow for vertical movement of the rod without impairing the vacuum inside the envelope 10. As shown in FIG. 1, the bellows 20 is secured by means of suitable sealed joints at its respective opposite ends to the operating rod 18a and the end cap 13.

Coupled to the lower end of the operating rod 18a, suitable actuating means (not shown) is provided. This actuating means is capable of driving the electrode 18 downwardly out of engagement with the electrode 17 so as to open the interrupter and is also capable of returning the electrode 18 to its illustrated position so as to close the interrupter. A circuit opening operation will soon be explained in greater detail.

Each of the electrodes 17 and 18 comprises a short tubular member 25 of a high conductivity metal, such as copper. Each of these tubular members 25 has a pair of end surfaces 26 and 27 extending transversely of the longitudinal axis of the tubular member and located in spaced apart relationship along the longitudinal axis. One of the end surfaces 26 faces the opposite electrode and constitutes an arc-running surface, as will soon appear more clearly. The other end surface 27 is referred to hereinafter as the back surface of the tubular member 25.

Brazed to the arc-running surface 26 of each electrode is a pair of circumferentially-spaced contact-making projections 30 and 31. When the electrodes are closed, the contact-making projections 30 and 31 on one electrode engage the contact-making projections 30 and 31, respectively, on the other electrode. This engagement between the two pairs of contact-making projection 30, 30 and 31, 31 provides two parallel paths through which current can flow between the contacts 17 and 18 when the interrupter is closed. It is to be understood that the projections 30 and 31 are formed of a suitable weld-resistant metal that permits the contacts to be readily separated during an opening operation.

When the electrode 17 is driven downwardly to separate the electrodes during an opening operation, an arc is initiated between the portions 30 or 31 of the two electrodes that last part. As will soon be explained in greater detail, this arc is immediately driven at high speed along the arc-running surface 26, and this facilitates interruption at an early current zero. In FIG. 2, such an arc is shown at 32 in an instantaneous position through which it passes during this movement about arc-running surface 26. Since the arc is initiated at the contact-making portions 30 or 31, these projections are occasionally referred to hereinafter as arc-initiating portions of the electrodes.

As shown in FIGS. 1 and 2, each of the tubular members contains a slot 28 which extends between the end surfaces 26 and 27. This slot 28 intersects the arc-running surface 26 at a point that is circumferentially spaced a substantial distance d from the point at which the slot intersects the back surface 27. The depth of the slot 28 is such that it extends across the entire radial thickness 2? of the tubular member 25. Accordingly, the two por-' tions of tubular member 25 on opposite sides of the slot 28 are completely separated by the slot.

For supporting each of the slotted tubular members 25 so as to prevent its being deformed by the forces incident to operation of the interrupter, a back plate 34 is provided adjacent but slightly spaced from the back surface 27 of each tubular member. This back plate 34 is preferably of a high conductivity material such as copper. The two back plates 34 are joined to the two conductive rods 17a and 18a respectively by a suitable joining process, such as brazing, that provides a low-resistance, mechanically-strong joint between the joined parts. Each of the tubular members 25 is connected to its back plate 34 by means of a plurality of circumferentially-spaced pins, each of which is brazed at one end to the tubular member 25 and at the other end to the back plate 34.

' In each electrode, only one of these pins is of a high conductivity material such as copper, and the others are of a very low conductivity material, such as stainless steel. Referring to FIG. 2, the copper pin is designated 40 and the other pins are designated 42. For reasons which will soon be explained, the copper pin 40 has a cross-sectional area much larger than that of the other pins 42, for example, ten times as large. FIG. 3 is an enlarged cross-sectional view showing a preferred construction for connecting the copper pin 40 between the tubular member 25 and its back plate 34. As shown in FIG. 3, the pin 40 has a reduced diameter portion at one end that fits into an opening 43 in the back plate 34. At its opposite end, the pin 40 has a slot therein that receives an edge of the tubular member 25. Brazing material is interposed between all the juxtaposed surfaces of the pin 40 and the other two parts 35 and 24 to bond the pin to these two parts.

FIG. 4 shows a preferred construction for connecting each of the stainless steel pins 42 between the tubular member 25 and the back plate 34. Here the tubular member 25 and the back plate 34 contain aligned openings 45. The stainless steel pin 42 has reduced diameter portions at its opposite ends that fit into these openings 45;

and brazing material is interposed between all the juxtaposed surfaces to bond these parts together.

In each contact, the copper pin 40 carries a major portion of the total current flowing between the back plate 34 and the tubular member 25. This is the case because copper has a conductivity approximately 50 times that of stainless steel, the material of which the other pins 42 are made, and because the cross-sectional area of the copper pin is many times greater than that of each of the stainless steel pins 42. Since most of the current entering the tubular member 25 from the back plate 34 enters through the pin 40, the joint between the pin 40 and the tubular member 25 is occasionally referred to herein as the entry region.

In each contact, the copper pin 40 is so located with respect to the slot 28 that any point on the are running surface 26 that is circumferentially aligned with the joint between pin 40 and tubular member 25 is on the opposite side of the slot 28 from this joint. Accordingly, the slot 28 will block current from flowing between the pin 40 and any point on the are running surface 26 that is circumferentially aligned therewith except by a path that has a circumferentially-extending component. Thus, the path of the net current between the pin 40 and an arc at any point on the arc-running surface 26 will have a component extending in a circumferential direction.

By forcing the net current flowing between the pin 40 and an are at any point on the arc-running surface 26 to follow a path that has a component extending in a circumferential direction, there is created for every arc position on the arc-running surface 26 a loop circuit that has its arms interconnected by the arc and extending effectively circumferentially with respect to the arc. For example, referring to FIG. 2, it will be seen that the current flowing through the are 32 follows a loop-shaped path L having its arms (shown in dot-dash lines) extending circumferentially of the tubular member 25 with respect to the arc. A loop circuit of this general configuration is present irrespective of Whether the arc is located on the arc-running surface 26. Current flowing through a loop circuit has a magnetic effect which tends in a well-known manner to lengthen the loop, and because of the circumferentially extending disposition of the arms of the loop L, this magnetic effect is a circumferentiallyacting magnetic force that acts to drive the are 32 from its position of FIG. 2 circumferentially to the right on the arc-running surface 26. As viewed from the top of the contact shown in FIG. 2, this circumferential motion would be in a counter-clockwise direction. Thus, for every position of the arc-running surface 26, there will be a circumferentially-acting magnetic force for driving the arc in a counter-clockwise direction. This discussion assumes that arc-motion will lgin the amperian direction.

In view of their role in forcing the current to follow the required path for the above-described arc-rotation, the slot 28, pin 40 and the means for forcing the major portion of the current to flow through the pin 40 are occasionally referred to hereinfater as current-directing means.

Considering for the movement only the bottom contact 18, as the arc moves counterclockwise away from the location depicted in FIG. 2, it becomes further and further away from the copper pin 40; and this causes the current path between the pin 40 and the lower arc terminal to have a progressively greater circumferentiallyextending component. This results in a progressively greater circumferentially-acting magnetic force on the are as it moves counterclockwise from its position of FIG. 2. The greatest magnetic force on the arc will be present when the arc is moved counterclockwise on arerunning surface 26 to a point adjacent the left-hand edge of the slot 28 inasmuch as the current path between the pin 40 and the lower arc terminal then extends completely around the tubular member 25. The slot 28 is sufficiently narrow to enable the arc to be driven thereacross. Ac-

cordingly, the counterclockwise circumferentially-acting magnetic force developed when the arc reaches the left hand edge of the slot 28 will drive the arc across the slot, thus enabling arc motion to continue repetitively about the arc-running surface 26 until the arc is finally extinguished. An advantage in my arrangement is that the arc-rotating magnetic force is at a maximum when the arc reaches the slot 28, and this helps to force the arc across the slot.

From the above discussion, it will be apparent that the arc-rotating magnetic force resulting from the configuration of the current path through the lower electrode 18 is weakest when the arc is just to the right of the slot 28, as shown in FIG. 2. In a preferred form of my invention, I compensate for this weakness by locating the slot of the upper electrode at an approximately diametrically-opposed location to the slot of the lower electrode. Hence, when the current path through the lower electrode has a relatively small circumferentiallydirected component, as shown in FIG. 2, the current path through the upper contact has a relatively large circumferentially-directed component. correspondingly, when the current path through the upper contact has a relatively small circumferentially-directed component, the current path through the lower contact has a relatively large circumferentially-directed component.

The above-described motion of the are about the arcrunning surface 26 is advantageous in that it lessens the amount of electrode material that will be vaporized by the arc and also increases the degree of diffusion of the vapors that are generated. This enables the vacuum to recover its dielectric strength at an increased rate and thereby improve the ability to prevent reestablishment of the are after a current zero, thus increasing the interrupting capacity of the interrupter.

It is important to note that the arc-initiating regions 30 and 31 of the electrodes are located at points where there is a relatively large circumferentially-acting magnetic force on the arc. For example, considering the current path between the pin 40 and each of the arcinitiating regions of the lower electrode, the arc-initiating region 30 is effectively displaced by about 360 from the pin 40, and the arc-initiating region 31 is effectively displaced by about 180 from the pin 40. Accordingly, an arc initiated at either of these two points will from its very inception be subjected to a high circumferentiallyacting magnetic force that quickly acts to drive it off the arc-initiation region and along the arc-running surface 26. No delay is required to allow time for the arc to be moved into a position for circumferential motion. This high speed initiation of circumferential arc-motion is desirable in reducing arc erosion of the contact-making regions 30 and 31 and also in reducing the total amount of vapors generated.

It will be noted that my basic contact structure is of a simple construction that can be readily formed inasmuch as it requires merely a short tubular member 25 with only a singleslot 28 therein. This slot 28 can be of a simple straight-line configuration and is therefore easily formed, as by a sawing operation.

Another advantage of the illustrated contact structure is its ability to withstand high mechanical forces without damage. Because the tubular element 25 contains only a single slot (28), there are no mechanically weak fingers or petals that are susceptable to damage by the high forces incident to operation of the interrupter. The pins 40 and 41 provide a mechanically strong bond between the back plate 34 and the tubular member 25 at points spaced about the entire circumference of the tubular member, thus reinforcing the tubular member and assuring against its distortion.

Although I prefer to form both of the electrodes as illustrated, my invention in its broader aspects comprehends an arrangement in which only one of the electrodes is so formed. Such an arrangement is illustrated in FIG.

5, where the upper electrode is of a form identical to the electrode 17 of FIGS. 1 and 2, but the lower electrode is in the form of a planar disk 18 having outer peripheral portions abutting the contact-making portions 30 and 31 of the upper electrode. The arc-rotating magnetic forces developed with this electrode arrangement of FIG. 5 are lower than in the preferred form, but this can be tolerated in certain ratings of the interrupter.

The arc-rotating magnetic forces can be increased in the illustrated electrodes by forcing an even greater percentage of the total current to flow through the pin 40. One way of increasing this percentage is to provide slots in the tubular member 25 adjacent to and aligned with each of the stainless steel pins 42. One such slot is illustrated at 50 in FIG. 6. These slots 50 force any current flowing through the stainless steel pins 40 and the tubular member 25 to follow a tortuous path that is longer and more restricted than the path available without the slots 50. This increases the resistance of this path and forces more current to flow through the copper pin 40, as is desired.

For protecting the insulation of a vacuum interrupter from the buildup of a metallic coating thereon as a result of arc-generated vapors condensing thereon, it is customary to provide a vapor-condensing shield between the arcing region of the interrupter and the protected insulating surface. Such a shield is shown at 52. This shield preferably corresponds to a similar shield shown and claimed in Patent 2,892,911, Crouch, assigned to the assignee of the present invention.

It is to be understood that in constructing the disclosed vacuum interrupter, the various parts inside the vacuum envelope should be freed of sorbed gases and other contaminants sufliciently to avoid harmful impairment of the vacuum by any such gases or contaminants during operation of the interrupter. Conventional vacuum processing techniques can be used for this purpose.

While I have shown and described particular embodiments of my invention, it will be obvious to those skilled in the art that various changes and modifications may be made without departing from my invention in its broader aspects; and I, therefore, intend in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An electric circuit interrupter comprising:

(a) a pair of electrodes between which an arc is developed during circuit interruption,

(b) one of said electrodes comprising a tubular member of relatively high conductivity metal,

(c) said tubular member having a pair of end surfaces extending transversely of the longitudinal axis of the tubular member and located in spaced-apart relationship along said longitudinal axis,

(d) one of said end surfaces facing the other of said electrodes and constituting an arc-running surface for arcs developed between said electrodes, the other end surface constituting a back surface,

(e) current-directing means for forcing the net current flowing through said tubular member to an arc terminal located at any point on said arc-running surface to follow a path through said tubular member that has a component extending circumferentially of said tubular member,

(f) said current-directing means comprising:

(1) a slot in said tubular member extending between said two end surfaces and intersecting the respective end surfaces at points circumferentially spaced from each other,

(2) and means for forcing the major portion of the current flowing through said tubular member to an arc terminal at any point on said arcrunning surface to enter said tubular member through an entry region located adjacent the point where said slot intersects said back surface and on the opposite side of said slot from the location of any point on said are running surface that is circumferentially aligned with said entry region,

(g) and means for supporting said tubular member comprising a plurality of elements of relatively low conductivity material circumferentially spaced about said tubular member and joined to said tubular member at said back surface.

2. An electric circuit interrupter comprising:

(a) a pair of electrodes between which an arc is developed during circuit interruption.

(b) one of said electrodes comprising a tubular member of relatively high conductivity metal,

(c) said tubular member having a pair of end surfaces extending transversely of the longitudinal axis of the tubular member and located in spaced apart relationship along said longitudinal axis,

((1) one of said end surface facing the other of said electrodes and constituting an arc-running surface for arcs developed between said electrodes, the other end surface constituting a back surface,

(e) current-directing means for forcing the net current flowing through said tubular member to an arc terminal located at any point on said arc-running surface to follow a path through said tubular member that has a component extending circumferentially of said tubular member.

(f) said current-directing means comprising:

(1) a slot in said tubular member extending between said two end surfaces and intersecting the respective end surfaces at points circumferentially-spaced from each other,

(2) a conductor of relatively high conductivity metal for supplying current to said tubular member,

(3) means for joining said conductor to said tubular member in an entry region located adjacent the point where said slot intersects said back surface and on the opposite side of said slot from the location of any point on said arcrunning surface that is circumferentially aligned with said entry region,

(4) and means for forcing a major portion of the current flowing through said tubular member to an arc terminal on said arc running surface to enter said tubular member through said entry region,

-(g) and means for supporting said tubular member comprising a plurality of elements of relatively low conductivity material circumferentially-spaced about said tubular member and joined to said tubular member at said back surface.

3. The circuit interrupter of claim 2 in which said tubular member contains slot means adjacent one of said low conductivity elements and in circumferential alignment therewith for forcing current flowing through said low conductivity element to follow a tortuous path in the portion of the tubular member adjacent said low conductivity element.

4. An electric circuit interrupter comprising:

(a) a pair of electrodes between which arcs are developed during circuit interruption,

(b) one of said electrodes comprising a tubular member having a pair of end surfaces extending transversely of the longitudinal axis of the tubular member and located in spaced apart relationship along said longitudinal axis,

(c) one of said end surfaces facing the other of said electrodes and constituting an arc-running surface along which an are developed between said electrodes can run, the other end surface constituting a back surface,

(d) current-directing means for forcing the net current flowing through said tubular member to an arc terminal located at any point on said arc-running surface to follow a path through said tubular member that has a component extending circumferentially of said tubular member,

(e) said current-directing means comprising:

(1) a slot in said tubular member extending between said two end surfaces and intersecting the respective end surfaces at points circumferentially spaced from each other,

(2) and means for forcing the major portion of the current flowing through said tubular member to an arc terminal at any point on said arcrunning surface to enter said tubular member through an entry region located adjacent the point where said slot intersects said back surface and on the opposite side of said slot from the location of any point on said arc-running surface that is circumferentially aligned with said entry region.

5. The circuit interrupter of claim 4 in combination with arc-initiating means for initiating any arcs developed between said electrodes at a loction that is circumferentially displaced from said entry region by at least of the circumference of said tubular member.

6. The circuit interrupter of claim 4 in combination with arc-initiating means for initiating any arcs developed between said electrodes at a location that is circumferentially spaced from said entry region by at least of the circumference of said tubular member.

7. The circuit interrupter of claim 4 in combination with means for making contact between said electrodes at a location near said slot but on the opposite side of said slot from said entry region.

8. The circuit interrupter of claim 1 in which both of said electrodes are constructed as defined in claim 1, the slots in said tubular members being circumferentially offset from each other by a substantial angular displacement.

References Cited by the Applicant UNITED STATES PATENTS 2,949,520 8/ 1960 Schneider. 2,964,679 12/ 1960 Schneider et al. 3,089,936 5/1963 Smith.

KATHLEEN H. CLAFFY, Primary Examiner. 

1. AN ELECTRIC CIRCUIT INTERRUPTER COMPRISING: (A) A PAIR OF ELECTRODES BETWEEN WHICH AN ARC IS DEVELOPED DURING CIRCUIT INTERRUPTION, (B) ONE OF SAID ELECTRODES COMPRISING A TUBULAR MEMBER OF RELATIVELY HIGH CONDUCTIVITY METAL, (C) SAID TUBULAR MEMBER HAVING A PAIR OF END SURFACES EXTENDING TRANSVERSELY OF THE LONGITUDINAL AXIS OF THE TUBULAR MEMBER AND LOCATED IN SPACED-APART RELATIONSHIP ALONG SAID LONGITUDINAL AXIS, (D) ONE OF SAID END SURFACES FACING THE OTHER OF SAID ELECTRODES AND CONSTITUTING AN ACR-RUNNING SURFACE FOR ARCS DEVELOPED BETWEEN SAID ELECTRODES, THE OTHER END SURFACE CONSTITUTING A BACK SURFACE, (E) CURRENT-DIRECTING MEANS FOR FORCING THE NET CURRENT FLOWING THROUGH SAID TUBULAR MEMBER TO AN ARC TERMINAL LOCATED AT ANY POINT ON SAID ARC-RUNNING SURFACE OF FOLLOW A PATH THROUGH SAID TUBULAR MEMBER THAT HAS A COMPONENT EXTENDING CIRCUMFERENTIALLY OF SAID TUBULAR MEMBER, (F) SAID CURRENT-DIRECTING MEANS COMPRISING: (1) A SLOT IN SAID TUBULAR MEMBER EXTENDING BETWEEN SAID TWO END SURFACES AND INTERSECTING THE RESPECTIVE END SURFACES AT POINTS CIRCUMFERENTIALLY SPACED FROM EACH OTHER, (2) AND MEANS FOR FORCING THE MAJOR PORTION OF THE CURRENT FLOWING THROUGH SAID TUBULAR MEMBER TO AN ARC TERMINAL AT ANY POINT ON SAID ARCRUNNING SURFACE TO ENTER SAID TUBULAR MEMBER THROUGH AN ENTRY REGION LOCATED ADJACENT THE POINT WHERE SAID SLOT INTERSECTS SAID BACK SURFACE AND ON THE OPPOSITE SIDE OF SAID SLOT FROM THE LOCATION OF ANY POINT ON SAID ARC RUNNING SURFACE THAT IS CIRCUMFERENTIALLY ALIGNED WITH SAID ENTRY REGION, (G) AND MEANS FOR SUPPORTING SAID TUBULAR MEMBER COMPRISING A PLURALITY OF ELEMENTS OF RELATIVELY LOW CONDUCTIVITY MATERIAL CIRCUMFERANTIALLY SPACED ABOUT SAID TUBULAR MEMBER AND JOINED TO SAID TUBULAR MEMBER AT SAID BACK SURFACE. 